Not Applicable.
Not Applicable.
This invention relates to an hydraulic variable camshaft timing (xe2x80x9cVCTxe2x80x9d) system for an internal combustion engine. More particularly, this invention relates to a system of the foregoing character with a moveable locking pin to lock the relative positions of a rotor attached to a rotating camshaft and a surrounding rotatable housing, which is otherwise relatively oscillatable with respect to the camshaft, during periods of low engine oil pressure and when an engine control system is operating to prevent relative oscillation between the camshaft and the surrounding housing.
Commonly assigned U.S. Pat. No. 6,250,625 describes an hydraulic VCT system of the self-powered type, that is, a type in which relative oscillating movement between a rotor secured to a rotating camshaft and a rotatable housing that surrounds the camshaft rotor is actuated by torque pulsations in the camshaft as the camshaft alternatingly opens and closes engine intake or outtake valves. The disclosure of the aforesaid U.S. Pat. No. 6,250,625 is incorporated by reference herein.
As is disclosed in the foregoing reference, it is desirable to prevent relative oscillation between the camshaft rotor and surrounding housing during periods of low engine oil pressure. To that end, the aforesaid reference teaches the use of an annular locking plate that rotates with the camshaft and is axially moveable relative to the camshaft and the surrounding housing to move into or out of engagement with the housing. Such movement serves to prevent relative oscillating movement between the housing and the camshaft when the locking plate is in engagement with the housing. The locking plate is biased away from locking engagement by engine oil pressure that acts on a surface thereof, and is spring biased into engagement during periods of normal operations by the biasing force of a spring acting on an opposed surface of the locking plate, the oil pressure being sufficient to overcome the biasing force of the spring to keep the locking plate out of its locking position during such periods of normal operation; however, during engine start-up or other periods of low engine oil pressure, the force of the biasing spring will overcome the opposed force of the engine oil, and will move the locking plate into its locking position.
Commonly assigned, co-pending U.S. patent application Ser. No. 09/488,903, now U.S. Pat. No. 6,311,655 B1, the disclosure of which is also incorporated by reference herein, also discloses an hydraulic VCT system with an arrangement to prevent relative oscillation between a rotating camshaft, specifically, a vane-carrying rotor that is secured to the camshaft, and a rotating housing that surrounds the camshaft rotor during periods of low engine oil pressure. The VCT system of the ""903 application is a system that relies on engine oil pressure for its actuation, rather than camshaft torque pulsations, to cause relative oscillation between the camshaft and the housing, and it relies on a slidable locking piston carried by a lobed rotor attached to the camshaft to slide a locking pin into a position in engagement with the housing during periods of low engine oil pressure.
Other patents that disclose various other hydraulic VCT arrangements for preventing relative oscillation between a camshaft and a surrounding housing during periods of low engine oil pressure include U.S. Pat. Nos. 6,053,138 (Trzmiel et al.), 4,858,572 (Shirai et al.) and U.S. Pat. No. 5,797,361 (MiKame et al.).
The present invention relates to a VCT system, either of the cam torque actuated (xe2x80x9cCTAxe2x80x9d) type or the engine oil pressure actuated (xe2x80x9cOPAxe2x80x9d) type, in which the positions of the relatively oscillating camshaft rotor and a surrounding housing can be locked when desired, even during normal operating conditions when engine oil pressure is relatively high. The camshaft rotor carries a slidable pin, which is slidable into and out of locking position with respect to the housing, and the sliding action of the slidable pin is controlled, not strictly as a function of engine oil pressure, but by the position of a control spool valve that is slidable along its axis to selectively control flow into and out of advance and retard chambers of the housing.
The control spool valve of the present invention has a centered or null position in which flow into and out of the advance and retard chambers is blocked. At the null position of the spool valve, however, a separate passage that contains the locking pin, which is spring biased towards its locking position and is subject to an opposing hydraulic force to urge it to its unlock position, is depressurized, which results in the locking of the rotor and the housing elements relative to one another. When the spool valve is on one side or another of its null position, the locking pin passage is pressurized to move the pin to its unlock position, at least during periods of adequate engine oil pressure, and oil will flow into one of the advance and retard chambers, and out of the other, to thereby lead to a phase change between the camshaft rotor and the surrounding housing. Thus, the rotor and housing are always positively locked in position relative to one another when there is no need to change the phase therebetween, which is the condition in which the engine control system controls the spool valve to maintain it at its null position. The locking of the positions of the rotor and housing relative to one another can occur at any of many potentially relative positions therebetween, depending on when the control system operates to reposition the spool valve to its null position.
Accordingly, it is an object of the present invention to provide an improved hydraulic VCT system. More particularly, it is an object of the present invention to provide a VCT system in which the relative positions of a camshaft rotor and a surrounding housing are positively locked when the control system is operating to control such elements without relative oscillating motion therebetween.
For a further understanding of the present invention and the objects thereof, attention is directed to the drawing and the following brief description thereof, to the detailed description of the preferred embodiment and to the appended claims.